Tomato: Responses to the spectrum at equal photon irradiance.
Light Actions
Figure 1: Range of wavelengths sensed through different higher-plant photoreceptor families (black “information acquisition”) or driving photoreactions (orange “energy dependent”). DNA damage and repair, photosynthesis and photoreceptors; CRY: cryptochromes 1 and 2, PHOT: phototropins 1 and 2, ZTL: Zeitlupe, UVR8: “UV-B” photoreceptor, PHY: phytochromes A/B/C/D/E.
Light Screening and Absorption
Figure 2: Optical properties of leaves (green) and protective light-screening pigments (dark red). Thick lines indicate high absorption and paler narrower lines moderate absorption by pigments, or absorptance, reflectance or transmittance for whole leaves.
Energy flow: electricity to produce
flowchart TB
E(**Electricity**) ==> LED([LED + driver])
LED == 30 to 50% ==> L([Light<br/>_irradiance and spectrum_*])
LED -- 50 to 70% --> H[[Heat]]
L == 1 to 100% ==> IL[Intercepted by leaves<br/>_leaf area index or LAI by imaging_*]
L -- 100 to 1% --> NIL[Not intercepted by leaves<br/>_reaches soil, pots, shelves..._]
NIL -- 100% --> WSA[Absorbed by objects] --> Hobj[[Heat]]
IL == 70 to 90% (LEDs) ===> Ab[Absorbed]
IL -- 5 to 20% (LEDs) --> Rfl[[Reflected<br/>_imaging_*]]
IL -- 2 to 15% (LEDs) --> Tfr[[Transmitted<br/>_imaging?_*]]
Ab ==> Ph[Photosynthesis] == 2 to 5%<br/>of absorbed ==> Ch([Chemical energy<br/>_metabolites_])
Ab --> Fl[[Light<br/>_fluorescence_*]]
Ab --> He[Heat] --> Tleaf([_temperature of leaves_*])
Tleaf <--> Cv[[Convection<br/>_cooling or warming_]]
Tleaf --> Tr[[Transpiration<br/>_cooling_]]
Tr -- water use --> Sw([Soil water<br/>_pot weight_*])
Ch == 0 to 70% (?) ==> Gr[Growth<br/>_plant size and shape_*]
Ch -- 100 to 30% (?) --> Mn[[Maintenance<br/>_defence and repair_]]
Cv <--> Tair([Air temperature*])
Tr <--> Hair([Air water vapour*])
Tleaf <--> Thr[[Thermal radiation<br/>_cooling or warming_]]
Gr == ?% ==> Hv[[**Harvested**<br/>**produce**]]
Gr -- ?% --> Wst[[Waste]]
How does regulation affect growth rate?
Starting with a given amount of “building blocks” (sugars + minerals) decisions involve:
grow big roots and small shoots, or vice versa,
make large thin leaves or smaller thick leaves,
continue making only leaves, or start making “fruits”,
grow tall to avoid shade from neighbours or stay low and tolerate the shade.
The compromise
A larger surface of leaves intercepts more light driving a faster growth rate (similar to compound interest), but to a point.
Larger/longer roots have access to more water and minerals allowing building new leaves, so a balance is needed.
What is a good balance depends on light irradiance and soil conditions.
“Decisions by the plant” are based to a significant extent on the colour of light, because in nature the colour of light informs about future shade.
Figure 3: Time course of PAR photon irradiance during four consecutive days with different cloud conditions. Measured with broad band-sensors at Viikki, Helsinki, Finland.
Figure 4: Time series of scaled spectral photon irradiance during one day. Spectra scaled to \(Q_{\lambda 400..700 nm} = 1000 \mu mol\,s^{-1}\,m^{-2}\). Data from Andes Lindfors.